Formulas: P_d = (V_in − V_out) × I_out + V_in × I_q (heat is dissipated because a linear regulator burns off the voltage difference as heat, unlike a switching regulator). Max allowable θ_JA = (T_j,max − T_a) ÷ P_d — the total junction-to-ambient thermal resistance (package + any heatsink) your cooling solution must be at or below. If you enter your actual θ_JA, resulting T_j = T_a + P_d×θ_JA is checked against the max rating. θ_JA for a bare TO-220 in free air is typically ~50–65°C/W; a heatsink lowers this significantly — check your specific heatsink's datasheet for its θ_SA rating.